This invention relates to processes employing a plasma or glow discharge and, in particular, to plasma etching utilizing xenon as one component of the gas mixture in the plasma.
As known in the art, plasma etching involves complex chemical and physical processes, and a certain amount of luck, in producing desired results. The parametric balance in a glow discharge is delicate. Modifying one parameter, e.g. pressure, may produce several changes, of which only one or two are desired. For example, increased pressure can cause polymer deposition in an etch machine.
It is well known in the art to isolate devices on a semiconductor wafer by way of a trench surrounding the devices. In order to assure isolation, the trenches are relatively deep, e.g. on the order of 3 to 15 microns. It is also desired that the trench be relatively uniform in shape and have vertical or nearly vertical sidewalls.
For shallow grooves, i.e. less than 3 microns deep, it is relatively easy to obtain a groove of uniform depth with vertical or nearly vertical, smooth sidewalls. For trenches, it is more difficult to produce the desired geometry. Further, producing a trench should not entail an etch time of two or three times that required to etch a groove. However, as known in the art, it is difficult to increase etch rate without sacrificing uniformity, either in depth or in the shape of the sidewalls. Typically, high speed etches result in rough surfaces and trenches which may bow or have a barrel cross-section. In addition, it is important that the trenches have vertical or nearly vertical sidewalls since devices which have already been formed on the wafer are often critically spaced. Thus, the trench should not change size, shape or position with depth. This is sometimes referred to "loss of critical dimension".
In the prior art, CF.sub.3 Br has been proposed for etching silicon (J. Vac. Sci. Technol. 17(6), November/December 1980, p. 1341f and IBM Technical Disclosure Bulletin, Vol. 24 No. 9, February 1982, p. 4725f) or titanium (J. Electrochem. Soc.: SOLID-STATE SCIENCE AND TECHNOLOGY, Nov. 1977, p. 1766f) in a plasma reactor. A difficulty with this particular gas is the tendency to deposit polymer in the plasma reactor. The difficulty with polymer formation is that it changes the electrical characteristics of the chamber as well as the chemistry of the process being performed therein. In addition, the polymer coating can become a source of particle contamination on the wafer.
Another difficulty is that the etch profile is very dependent upon the flow of CF.sub.3 Br. This renders etch profile difficult to control. This problem can be acute, particularly for newer, high density structures. For example, megabit and higher density memories are constructed using what is known as a trench capacitor, which requires an anisotropic etch profile on small features in the monocrystalline silicon wafer. The control of geometry is particularly difficult on submicron features and deep trenches because ion-molecule collisions cause scattering of the ions, producing concave etch profiles and reduced etch rates. One proposed solution to the problem has been to use low pressure etching systems. However, etch rate and throughput are reduced in low pressure processes.
In view of the foregoing it is therefore an object in accordance with one aspect of the present invention to provide a process for forming trenches having a controlled etch profile.
A further object, in accordance with another aspect of the present invention, is to provide an improved plasma etch resulting in reduced polymer deposition within the plasma chamber.
Another object of the present invention is to provide an improved reactor having a sacrificial structure with erodes during the etch process to prevent polymer buildup in the reactor chamber.
A further object of the present invention is to provide a means for improving control of ion production and bombardment in a process using plasma.